Processes and time scales of dacite magma assembly and eruption at Tauhara volcano, Taupo Volcanic Zone, New Zealand

Abstract

Magma mixing plays a prominent role in the origins of intermediate magmas in subduction zones. However, the conditions and time scales of magma mixing and how these are linked to subsequent eruption are unclear. Mount Tauhara is the largest dacitic volcanic complex in the Taupo Volcanic Zone, New Zealand. Dacites from Tauhara volcano have a complex petrography (Qtz + Plag + Amph + OPx + CPx + Oxi ± Oli) that can only have been produced by magma mixing and offer an ideal opportunity to investigate the processes and time scales involved in assembling dacite magmas in a continental subduction zone. Here, we present whole‐rock and mineral‐specific major and trace element and isotopic data for the Tauhara dacites in order to identify the magma mixing end‐members, constrain the physical conditions of mixing, and estimate the time scales and relationships between magma mixing, ascent, and eruption. These data reveal that four separate mixing events between crystal‐rich rhyolites (77–80 wt % SiO2; 40 ppm Sr) and crystal‐poor mafic magmas of basaltic (48 wt % SiO2; 1340 ppm Sr) to andesitic (55–59 wt % SiO2; 490–580 ppm Sr) composition occurred to produce the Tauhara dacites. Mixing took place in well‐stirred magma chambers located at midcrustal depths (8–13 km) at temperatures from 840 to 900°C. The time scales of magma mixing obtained from Ti diffusion in quartz appear to be largely dependent on the temperature and viscosity contrast between the end‐members as andesite and rhyolite magma mixed on time scales of 2–7 months, whereas basalt and rhyolite magmas mixed on time scales of 1–2 years. The short magma mixing time scales, combined with the physical properties (e.g., viscosity and density) of the mixed dacite magmas, as compared with those of the end‐member magmas, facilitated the ascent and eruption of dacite magmas at Tauhara volcano.